This invention relates to a microcellular polyurethane polymer and a process for its preparation using an isocyanate-terminated poly(oxytetramethylene) glycol prepolymer.
Microcellular polyurethane polymer for applications such as, for example, shoe soles typically may be obtained by reaction of an isocyanate compound with an active hydrogen-containing compound such as, for example, a polyester polyol. For processing convenience advantageously, the isocyanate compound is an isocyanate-terminated prepolymer prepared by reaction of an excess of an aromatic diisocyanate with a low molecular weight diol such as, for example, dipropylene glycol and/or tripropylene glycol. Subsequently, such prepolymer is reacted with an active hydrogen-containing composition, frequently comprising a polyester or polyether polyol, in the presence of a physical blowing agent such as, for example, trichlorofluoromethane to provide the polyurethane. The preparation of polyurethane polymer by such procedures is described in, for example, patent publications E.P. 235,888; E.P. 175,733; U.S. Pat. Nos. 3,591,532; 3,901.959; 4,647,596 and 4,757,095.
Isocyanate-terminated prepolymers obtained, in the manner as described above, from low molecular weight polyols or diols are frequently identified as a "hard-segment" prepolymers in contrast to "soft-segment" prepolymers generally obtained from high molecular weight polyols or diols. The terminology, "hard-" and "soft-segment", derives from the morphology of elastomeric polymers which can contain distinct phase separated regions. Such regions can be detected by thermoanalysis techniques and distinguished by, for example, glass transition temperatures. Generally, soft-segments of the polymer can be considered as having glass transition temperatures below room temperature whilst hard-segments can be considered as having glass transition temperatures above room temperature or even melting points if a crystallite. It is the current opinion and hence their classification that "soft-segment" prepolymers are associated with the formation of the soft-segment phase of the elastomer and conversely hard-segment prepolymers with the hard-segment phase of the elastomer. Structure-property relationships of hard- and soft-segment phases are described for example by Redman in "Developments in Polyurethanes--I" J. M. Buist Ed., Elsevier, London--published 1978. The distinction of the prepolymer type on molecular weight of the polyol used in the preparation of the prepolymer is arbitrary but general such prepolymers obtained from diols or triols having an equivalent weight of about 150 or less are considered to be "hard- segment" prepolymers.
Although providing microcellular polyurethane polymers with commercially attractive physical properties such above-mentioned processes have a number of disadvantages. Firstly, suitable polyester polyols are costly and frequently highly viscous or even low-melting point solids thus requiring handling and processing at elevated temperatures. Secondly, the use of hard-segment prepolymer restricts and makes it difficult to substitute or eventually replace all of the physical blowing agent, with for example water, without affecting significantly the physical properties of the resulting polymer, especially its flexibility and abrasion resistance properties. Polyester polyol-based polyurethane polymers frequently exhibit poor low temperature flexibility, especially at temperatures below about -25.degree. C., thus limiting there use in some applications.
As an alternative to polyester polyols, cheaper polyether polyols which generally have lower viscosities and thus can be handled and processed at ambient temperature, may be used in the preparation of the microcellular polyurethane polymers. However, the physical properties of the resulting polymer may and often are inferior compared to polyester-based polymers in for example wear resistance.
Due to current environmental concern relating to the earths atmosphere it is highly desirable to substitute certain physical blowing agents with alternative blowing agents. A highly suitable alternative blowing agent is water. It is therefore desirable to develop a new process for the preparation of polyurethane polymers, particularly microcellular polyurethane polymers, based on polyether polyols allowing for convenient processing and use of a blowing agent comprising water. Particularly it is desired to provide such a process that leads to the manufacture of polyurethane polymers having desirable physical properties at low temperatures, e.g. below 0.degree. C. To this purpose we have investigated the use of "soft-segment" prepolymers in the preparation of polyether polyol-based polyurethane polymers and more particularly the use of "soft-segment" prepolymers obtained by reaction of methylene diphenylisocyanate with a poly(oxytetramethylene)glycol. Such types of prepolymer and their use in the preparation of resilient polyurethane elastomers have been documented in the literature see, for example, U.S. Pat. No. 4,739,027 which discloses the reaction of prepolymer with a chain extending composition containing low molecular weight triol and diol components.